[RFA v2 0/3] Handle dereferencing Rust trait objects

[Tom Tromey <tom@tromey.com>]

This is version 2 of the patch for inspecting Rust trait objects.

I think this version addresses the comments from the earlier version:

* Use a std::vector rather than unordered_map -- note that a temporary
  set was needed to avoid temporarily making a gigantic vector in the
  libxul case.

* Change a reinterpret_cast to a static_cast.

* Use readelf to check the test's debuginfo before proceeding.

I've also tacked on a couple of patches that we discussed; one to
change the "subclass" information for struct symbol to use an enum,
and one to change template_symbol to use derivation.

Tested on x86-64 Fedora 25.

Tom